Anti-Occupy Central protesters stand behind a police cordon on Nathan Road at Hong Kong's Mongkok shopping district October 3, 2014, demanding the tents set up by pro-democracy protesters to be demolished.

Fresh scuffles broke out on Saturday between Hong Kong pro-democracy protesters and opponents of the week-long demonstrations, reigniting concerns that the Chinese-controlled city's worst unrest in decades could take a violent turn.

The protests have been largely peaceful since police last Sunday fired tear gas at crowds demanding Beijing grant Hong Kong the unfettered right to choose its own leader. But the mood turned ugly on Friday at the most volatile protest site, in the teeming suburb of Mong Kok where some criminal gangs are suspected to be based.

Police intervened to prevent a violent escalation, but a rowdy crowd of around 2,000 filled a major intersection in the small hours of Saturday and the atmosphere was highly charged as police in riot gear tried to keep them under control.

Witnesses said about a thousand protesters faced off in Mong Kok at mid-morning on Saturday, although there were no uniformed police in sight.

Pro-democracy activists vowed to hold their ground through the weekend. "We will retreat after that if the situation gets worse, such as if mobs start flashing their knives at us," said Daniel Tang, who is in his 30s.

Student activists, established protest groups and ordinary Hong Kongers have joined forces to present Beijing with one of its biggest political challenges since it violently crushed pro-democracy protests in and around Tiananmen Square in 1989.

Tens of thousands of protesters have staged sit-ins across Hong Kong over the past week, demanding the city's pro-Beijing leader Leung Chun-ying step down and China reverse a decision in August to handpick the candidates for a 2017 election.

Police said they had arrested 19 men since Friday, with local state-owned broadcaster RTHK saying eight of them were suspected members of Triads, or Chinese criminal gangs. Eighteen people were injured, including six police officers, according to the RTHK report.

One of the main student groups behind the "Occupy Central" protest movement said it would pull out of planned talks with the Hong Kong government, because it believed authorities had colluded in the attacks on demonstrators in Mong Kok.

"The government and police today connived in the attack by Triads ... on peaceful occupiers, so they have shut the way to dialogue and must bear the consequences," the Hong Kong Federation of Students said in a strongly worded statement.

The notorious Triads operate bars, nightclubs and massage parlors across Mong Kok, an area of high-rise apartment blocks across the harbor from the main protest areas.

Witnesses said anti-Triad police wearing trademark black vests were active in the area on Friday.

At times over the past week, police have left the streets, saying they wanted to ease tensions, though the reason for their apparent absence from this scene on Saturday morning was unclear.

Police have defended their handling of fighting in the area, saying they had exercised "dignity and restraint and tried our best to keep the situation under control".

But Amnesty International issued a statement criticizing them for "(failing) in their duty to protect hundreds of peaceful pro-democracy protesters from attacks by counter demonstrators."

Protests "but a daydream"

The ruling Communist Party's official People's Daily, in a front page editorial on Saturday, praised Hong Kong police for their restraint in the face of what it said was lawless protests, including "poking" of police with umbrellas.

"A democratic society should respect the opinions of the minority, but it does not mean those minorities have the right to resort to illegal means," it wrote.

The protests will never spill over into the rest of China, the newspaper added.

"For the minority of people who want to foment a 'color revolution' on the mainland by way of Hong Kong, this is but a daydream."

Facing separatist unrest in far-flung and resource-rich Tibet and Xinjiang, Beijing is standing firm on Hong Kong, fearful that calls for democracy there could spread to the mainland, especially if successful.

Standoff

Demonstrations across Hong Kong have ebbed and flowed since last Sunday, when police used pepper spray, tear gas and batons to break them up in the worst unrest in Hong Kong since the former British colony was handed back to Chinese rule in 1997.

At times, tens of thousands of people gathered to block roads and buildings in central areas, bringing them to a virtual standstill.

Earlier this week, Leung rejected protesters' demands to resign, and he and his Chinese government allies made clear they would not back down.

He did, however, offer talks with leaders of the movement.

China rules Hong Kong through a "one country, two systems" formula underpinned by the Basic Law, which accords Hong Kong some autonomy and freedoms not enjoyed on the mainland and has universal suffrage as an eventual goal.

But Beijing decreed on Aug. 31 it would vet candidates who want to run for chief executive at an election in 2017, angering democracy activists, who took to the streets.